Freedoom is a project to create a complete Doom II-compatible
IWAD file which is Free Software.
The IWAD file is the file used by Doom which contains all the
game data (graphics, sound effects, music, etc.). While the
Doom source code is Free, you currently still need one of the
proprietary IWAD files from id in order to play Doom. Freedoom
aims to create a Free alternative. Combined with the GPL-licensed
Doom source code this will result in a complete Free Doom-based
game.
For more information, see http://freedoom.sf.net/